Materials Science Forum Vols. 821-823

Abstract: This work reports on the morphological and electrical characteristics of Ni/4H-SiC Schottky contacts, fabricated on epitaxial layers intentionally covered by micrometric size Ge-droplets. Specifically, the Ge-droplets behave as preferential paths for the vertical current conduction, as observed at nanometric scale by conductive atomic force microscopy. As a consequence, the electrical I-V characteristics of these Ni contacts revealed the presence of a double-barrier, thus indicating an inhomogeneity in the interface. This behavior was associated to the local Schottky barrier lowering contribution due to the Ge-presence. These results can be useful to explore the possibility of controlling the contact (Schottky or Ohmic) properties by changing the size and the distribution of the surface impurities.

Abstract: The mechanism of Ohmic contacts formation to p-type SiC is a fundamental and technological concern continuously under debate. Typically, Ti/Al-based contacts are a good choice for Ohmic contacts to p-type SiC, even though some aspects strictly related to the specific nature of Al (susceptibility to oxidation, low melting temperature, etc.) remain to be optimized. In this work, the evolution of the electrical properties of a Ti/Al/W multiple-layer contact has been studied by TLM characterization and correlated to the changes in the morphology and microstructure upon thermal annealing. The formation of an Ohmic contact has been observed after a thermal annealing at 1100°C, discussing the possible reasons determining the transition to an Ohmic behavior.

Abstract: In order to form Ti₃SiC₂ on 4H-SiC(0001) 8°-off, 200 nm of Ti₃₀Al₇₀ was deposited onto SiC substrates by magnetron sputtering from pure Ti₃₀Al₇₀ targets. The samples were then annealed at 1000°C for 10 min under Ar atmosphere in a Rapid Thermal Annealing (RTA) furnace. Structural analyses reveal the formation of epitaxial hexagonal Ti₃SiC₂ (0001) oriented. Elemental analyses show that high amount of Al and O elements are present inside the deposit. Obviously, the formation of Ti₃SiC₂ is accompanied by parasitic Al oxide, probably due to some unwanted oxygen residual in the RTA chamber. By using proper backing steps before the annealing, the deposit is not anymore composed of only Ti₃SiC₂ but accompanied with other compounds (Al₃Ti, and Al). On the oxide-free sample, the specific contact resistance ρ_c of the Ti₃SiC₂ based contact on p-type 4H-SiC (having N_a= 2×10¹⁹ cm^-3) was measured to be as low as 6×10^-5 Ω.cm².

Abstract: The electrical behavior and stability of a temperature sensor based on 4H-SiC Schottky diodes using Ni₂Si as Schottky contact, are investigated. The ideality factor and the barrier height were found to be strongly dependent on the post-annealing temperature of the Ni contact (which lead to the formation of Ni₂Si). A nearly ideal Schottky device, with the barrier height approaching the high value of 1.7eV, and a slight temperature dependence, was obtained after an annealing at T_A=800°C. This high barrier height proves that Ni₂Si is suitable as Schottky contact for temperature sensors, able to reliably operate up to 450°C. Sensor sensitivity levels between 1.00mV/°C and 2.70 mV/°C have been achieved.

Abstract: Ohmic CoSi₂ contacts to n-type 4H-SiC showing low contact resistance have been made by sputter depositing sequential layers of Si and Co on 4H-SiC substrates followed by a two-step rapid thermal anneal at 600 °C and 950 °C. The contacts formed have been characterized at temperatures ranging from-40 °C to 500 °C with a specific contact resistance of 3.80∙10^-5 Ωcm² at 25 °C and a minimum of 6.0∙10^-6 Ωcm² at 500 °C.

Abstract: The analysis of Ti/Ni metal-layer as Ohmic and Schottky contacts to 4H n-SiC (with a doping concentration of ~1E18 cm^-3) is reported. Both Ti (10nm/Ni (100nm) contact and Ti (20nm)/Ni (100nm) contact were found to have Ohmic behavior with comparable specific contact resistance (~4.3 to 5.3×10^-4 Ωcm²) after annealing at 1100 °C. Ti (10nm)/Ni (100nm) contact annealed at 500 °C and 600 °C was also demonstrated as Schottky contact to 4H n-SiC layers.

Abstract: We report nitrogen (N) doping of 4H-SiC by KrF excimer laser irradiation in liquid N₂. In comparison to phosphorus (P) doping performed using phosphoric acid solution, the liquid-N₂ immersion-laser doping can introduce N atoms deeper (~ 1 μm depth) into the 4H-SiC, which results in reduction of doped layer resistance by approximately 3 orders of magnitude. Doping is shown to proceed by the thermal diffusion of species, while loss of the host material from the surface by ablation takes place at the same time. Chemical analysis shows that high density carbon (C) vacancies are generated in the N doped region, which suggests enhanced diffusion of N assisted by the presence of C vacancies. pn junction diodes are formed by using the N doping technique. Turn-on voltage is ~ -3V, which is reasonable for a pn junction diode of 4H-SiC.

Abstract: – Thermal contact resistances between a silver metallized SiC chip and a direct bonded copper (DBC) substrate have been measured in a heat transfer experiment. A novel experimental method to separate thermal contact resistances in multilayer heat transfer path has been demonstrated. The experimental results have been compared with analytical calculations and also with 3D computational fluid dynamics (CFD) simulation results. A simplified CFD model of the experimental setup has been validated. The results show significant pressure dependence of the thermal contact resistance but also a pressure independent part.

Abstract: It was discovered that the oxidation rate for SiC depended on the conduction type. The oxidation was performed for SiC(0001) with nitrogen doping (n-type) in the range from 2×10¹⁶ cm^-3 to 1×10¹⁹ cm^-3, and aluminum doping (p-type) in the range from 2×10¹⁵ cm^-3 to 1×10¹⁹ cm^-3, exhibiting a clear dependence. For n-type SiC the oxide thickness increases for higher doping density, and for p-type the thickness decreases. Note that in the case of Si oxidation, there exists very little difference of oxidation rate between the conduction types in such low doping density, and the dependence is peculiar to SiC.

Abstract: We measured Fourier transform infrared (FT-IR) and cathodoluminescence (CL) spectra of SiO₂ films with various thicknesses, grown on 4H-SiC substrates. The appearance of broad phonon modes at ~1150–1250 cm^-1 in p-polarized light and their disappearance in s-polarized light confirmed that the phonon modes at ~1150–1250 cm^-1 originated from surface polaritons (SPPs). For the thin SiO₂ film (8-nm thick), the peak frequency of the transverse optical (TO) phonon in the SiO₂ film on the 4H-SiC substrate was observed at ~1080 cm^-1 and was higher than that in SiO₂ films on the Si substrate (1074 cm^-1). This suggested that the thin SiO₂ film (8-nm thick) is under compressive stresses at the interface between the SiO₂ film and SiC substrate. On the other hand, for the thick SiO₂ films (85 and 130-nm thick), the TO phonon peak frequency tended to shift toward lower frequencies with increasing oxide layer thickness. The CL measurement indicated that the CL peak intensity at ~640 nm, attributed to non-bridging oxidation hole centers (NBOHCs), became stronger with increasing oxide layer thickness, relative to that of the CL peaks at ~460 and 490 nm due to oxygen vacancy centers (OVCs). By comparing the FT-IR and CL measurements, we concluded that the TO phonon red-shift with increasing oxide layer thickness can mainly be attributed to an increase in inhomogeneity with increasing oxide layer thickness for the thick SiO₂ films.